A great variety of instruments for measuring penetrating electromagnetic radiation exist in the art, including Geiger-Muller counters, scintillation detectors, photographic emulsions and various ionization chambers. Each of these general types of radiation detectors have several realizations, the sensitivity of which in spectral response characteristics varies widely within the overall class. The capabilities of accurate measurement of any given instrument are always restricted, and no satisfactory general method has been devised for all applications. This accounts, in part, for the wide variety available.
Of the ionization chambers, the so called air wall cavity chambers are the most satisfactory. Such a chamber is one in which the volumetric ionization is independent of chamber volume, proportional to the gas density in the chamber, proportional to the photon energy absorbed per unit volume of wall material and inversely proportional to the stopping power of the wall material for secondary electrons. Walls composed of materials with atomic numbers close to the average for air satisfy these criteria. Examples of such materials are graphite, lucite, bakelite and beryllium.
A number of commercially available dosimeters have utilized the air wall cavity chamber principle for design of the radiation sensors. Integrating, or capacitive, types of chambers are in this category. Typically, an air wall capacitor of known capacitance is charged by applying a known voltage thereto. The charged capacitor is subsequently exposed to radiation, which ionizes part of the confined gas and slowly discharges the capacitor. After some exposure period, the remaining charge is measured electronically, typically with a high impedence electrometer, and the accumulated dose is deduced.
U.S. Pat. No. 3,984,690 by Marshall et al discloses a current measuring apparatus associated with such a capacitor to give a periodically updated dose rate based on current flow through the capacitor. The Marshall device employs an analog-to-digital converter which generates pulses at a frequency proportionally dependent on the value of the current through the ionization chamber.